1. Field of the Invention
The present invention relates to a method and an apparatus used in a process of dry-etching films on a wafer to form contact holes, including via holes and through holes, (hereinafter referred to as contact etching) in the manufacture of semiconductor devices, such as ULSIs.
2. Description of the Related Art
As one of the most fearful failures in contact etching, there is mentioned “etch-stop.” Etch-stop refers to a phenomenon in which etching stops while in progress.
Etch-stop can be roughly classified into two cases. The first case is a phenomenon in which the amount of seed depositions is excessively larger than the amount of ions contributing to etching and, therefore, the taper angle of a hole becomes large, which causes the hole to fail to penetrate through a film to-be-etched and which causes the hole's leading end to close. The second case is a phenomenon in which etching stops abruptly when etching is being performed in a favorable shape.
If such an etch-stop, as described above occurs all over the wafer plane, all of the devices formed on the wafer become defective.
In a mass-production plant, several thousand wafers are continuously transported within the reaction compartment (hereinafter referred to as the chamber) of a plasma treatment apparatus. In this case, the state of the chamber may change with time and etch-stop may result while several thousand wafers are being etched, even if conditions for deriving the best etching performance have been set in the initial phase of mass production. Specifically, the chamber state changes and etch-stop occurs because parts are worn, or because deposits are accumulated on the inner walls of the chamber.
Hence, currently, etching conditions allowing significant margins are set from the beginning of manufacture, so that etch-stop does not occur even if the chamber state more or less changes. In the case of insulating film etching, etch-stop occurs mostly due to the above-described first case wherein the amount of seed depositions in plasma becomes excessively large. Accordingly, as an example of a specific method, an oxygen flow rate is set approximately 5 sccm higher than the best flow rate condition. However, etch-stop due to the above-described second case infrequently occurs just because etching slightly deviates from favorable etching conditions. Hence, there has been a large need in mass-production sites for apparatus and recipes in which etch-stop never occurs even if the chamber state changes while the best etching conditions remain unchanged.
In addition, the performance required for contact etching has become increasingly higher along with miniaturization of semiconductor devices.
For example, requirements when forming contact holes having a high aspect ratio (depth/inner diameter) in a 60 nm-generation DRAM manufacturing process include an etching rate of 700 nm/min or higher, a selectivity to mask of 7 or higher, a selectivity to substrate of 50 or higher, a dimensional shift of 5 nm or less, a taper angle of 89° or larger, no bowing, no etch-stop, and so on. Here, bowing refers to a phenomenon in which the cross section of a sidewall within a hole forms into an arch. The most important of these requirements is “no etch-stop.” This is because the occurrence of etch-stop directly leads to a yield decrease, thereby causing a serious problem.
In the contact etching of such devices as described above, the etching rate, the selectivity to mask, the selectivity to substrate, the dimensional shift, the amount of bowing, and the like are sacrificed to a great extent, if the oxygen flow rate is set higher than the best flow rate condition for deriving necessary etching performance in order to avoid etch-stop. Therefore, it becomes less easy to secure etching conditions allowing margins at the time of mass production since the performance (specifications) required for etching becomes severer along with the advance in device miniaturization.
Accordingly, there is a strong need to consistently apply the best etching conditions to all wafers.
Note that Japanese Patent Laid-Open No. 2000-200772 (hereinafter referred to as Patent Document 1) proposes a method of preventing the occurrence of etch-stop by increasing the amount of O2 added to a treatment gas, either continuously or in stages, according to a change in the content of each constituent in plasma at the time of high-aspect ratio contact etching. However, this document does not describe at all the steps that need to be taken after etch-stop is avoided. If the additive amount of O2 that increased from the start of etching remains unchanged after the avoidance of etch-stop, it may become no longer possible to satisfy the specifications required for contact etching, including the etching rate, the selectivity to mask, the selectivity to substrate, the dimensional shift, and the amount of bowing.
In addition, Japanese Patent Laid-Open No. 11-195644 (hereinafter referred to as Patent Document 2) discloses a method of measuring the emission intensities of C, F and O in plasma during etching to adjust the concentrations thereof, for the same purpose as in Patent Document 1, so that the ratio among these constituents falls within such a correct range as not to cause etch-stop. However, this method requires that the flow rates of various gases be controlled in order to always maintain the emission intensities of C, F and O in plasma within correct ranges, thereby causing control configuration to become complicated.